1. Field of the Invention
The present invention relates to a sheet feeding unit, a sheet feeding apparatus, and an image forming apparatus for one-by-one feeding sheets that tend to stick to each other.
2. Description of the Related Art
In image forming apparatuses such as photocopiers and printers, cut sheets of paper that can be continuously fed are normally limited to sheets of high-quality paper and normal paper designated by photocopier manufacturers. In order to separate and feed such sheets one-by-one, various frictional separation techniques, for example, a retard roller separation technique and a separation pad technique have been used.
The retard roller separation technique will be described. A pickup roller is in contact with the uppermost sheet of a stack of sheets. A feed roller is provided on the downstream side of the pickup roller in the sheet feeding direction. A separation roller is in contact with the feed roller and driven at a predetermined torque in the opposite direction from the sheet feeding direction. Of the sheets sent out by the pickup roller, only one sheet passes the nip between the feed roller and the separation roller. Thus, double feeding is prevented.
In the separation pad technique, a friction member is pressed against a feed roller at a predetermined pressure, and only one sheet passes through the nip therebetween. Thus, double feeding is prevented.
For example, in the case of the retard roller separation technique, sheets can be fed one by one, by appropriately setting the torque and pressure of the separation roller in consideration of the frictional force between the sheets.
In accordance with the diversification of recording media, there is an increasing demand to form an image on, for example, a sheet of very heavy paper, an overhead projector (OHP) sheet, and a sheet of art film. In addition, in accordance with a growing need for color printing, there is also an increasing demand to form an image on a sheet of coated paper having a surface coated to increase whiteness or glossiness.
However, in the case of sheets that are formed of a resin material that tends to be electrically charged, such as OHP sheets and sheets of art film, in a dry environment, the surfaces of the sheets are gradually charged due to friction therebetween during a feeding operation. Therefore, due to the Coulomb force, the sheets can stick to each other. Therefore, failure in pickup or double feeding can occur.
In the case of coated paper, under high humidity, the sheets can stick to each other. Therefore, in the conventional separation techniques, failure in pickup or double feeding can occur. This is because, in the conventional separating techniques, only the frictional force between sheets is considered.
In the case of the above special sheets, the frictional force between sheets is equal to or less than that of normal paper. However, the adhesive force between resin sheets due to frictional charge in a dry environment and the adhesive force between sheets of coated paper under high humidity are much higher than the frictional force between the sheets. Therefore, in the conventional separation techniques, such special sheets can fail to be separated.
To eliminate the above-described strong adhesion between sheets, there is proposed a sheet feeding apparatus having an auxiliary air adhesion eliminating device that blows air against a stack of sheets from the side. This apparatus blows air against a stack of sheets from the side, thereby eliminating adhesion between the sheets in advance. After the adhesion between the sheets is eliminated, a pickup roller sends out the sheets. A separating section provided on the downstream side of the pickup roller separates one sheet from the other. This apparatus is used in the print industry and some photocopiers.
Unlike generally used apparatuses that use only the frictional separation technique, the feeding apparatus having an auxiliary air adhesion eliminating device can separate even the above highly adhesive sheets because it eliminates adhesion before feeding. Concerning such a feeding technique using an auxiliary air adhesion eliminating device, many proposals have been made, for example, Japanese Patent Laid-Open No. 11-005643 (corresponding to U.S. Pat. No. 6,015,144).
FIG. 16 shows an example of a sheet feeding apparatus 155 having an auxiliary air adhesion eliminating device. This sheet feeding apparatus 155 includes a feed tray 156 in which sheets S are loaded, and a sheet feeding device (not shown) that sends out sheets from the feed tray 156. The sheet feeding apparatus 155 further includes an air blowing device 71 serving as an auxiliary air adhesion eliminating device. The air blowing device 71 blows air against the side and over the top of the stack of sheets from the direction perpendicular to the side surface of the stack.
The sheet feeding apparatus 155 further includes an airflow moving device 157, which includes an electric motor 121 and cam plates 123. The motor 121 rotates the cam plates 123 so as to vertically move the air blowing device 71. Thus, the airflow is vertically moved.
Japanese Patent Laid-Open No. 2001-48366 discloses an apparatus including an auxiliary hot-air adhesion eliminating device capable of drying sheets by blowing air heated with a heater. This can weaken the adhesive force particularly between sheets of coated paper under high humidity.
However, the above sheet feeding apparatus using an auxiliary air adhesion eliminating device or an auxiliary hot-air adhesion eliminating device requires devices such as an air blowing device, a heater device, and an electric motor. Therefore, such a sheet feeding apparatus has been used in a relatively large feeding deck whose capacity is 2000 to 4000 sheets. Therefore, when being applied to apparatuses such as a photocopier, the feeding apparatus can be applied only to high-speed and high-class models to which a large feeding deck can be attached.
Therefore, the feeding apparatus cannot be applied to relatively low and medium class models to which a large feeding deck cannot be attached, and models for office use whose installation space is limited. If such models use highly adhesive sheets, the sheets can fail to be separated and double feeding can occur.